Method and bending device for bending flat metal workpieces

ABSTRACT

The invention relates to a method and device for bending flat metal workpieces, especially steel sheets, by bending freely over edges along a bending line with a straight, linear movement of the tool, wherein the bending edges are arranged parallel to the longitudinal direction of the sheet and the tool is displaced along the width of the metal sheet, characterized by the steps: inserting a punch between the bending edges of a die, holding the punch at the lowest point of the bend, applying compressive stress in order to level out the stresses in the shaping zone produced by the bending and incrementally superimposing the tensile stress with compressive stress, that is effective along the bending line in the direction of the sheet width of the bent workpiece, wherein the tool producing the compressive stress is moved along the bending line in the direction of the sheet width. In the bending device between the die assemblies below the punch, a carriage is arranged that can be displaced in the direction of the bending axis (i.e. in the sheet width) with a nip roller being effective in the punch axis, which has a groove in the plane of intersection with the punch axis.

The invention relates to a method for bending flat metal workpieces, inparticular sheet steel, by bending freely, also referred to asfree-bending or air-bending, over edges along a bending line with astraight linear movement of the tool.

The shaping is performed according to the invention by

a) inserting a punch between the bending edges of a forming die,

b) holding the punch at the lowest point of the bend,

c) applying compressive stress to superimpose the tensile stress in theshaping zone,

d) incrementally superimposing the tensile stress in the shaping zonewith compressive stress along the bending line of the workpiece to bebent.

The invention also relates to a device for performing the methodaccording to the above paragraph, consisting of a forming die with twoparallel die assemblies for holding the bending edges and a puncharranged above the forming die, wherein the die assemblies can beinterchanged for setting different die widths and underneath the punch atool carriage is arranged which can be moved in the direction of thebending line and comprises a nip roller which is aligned and adjustableagainst the workpiece to be bent.

The present invention relates to bending methods standardised by DIN8586 and in particular to the free bending of flat metal workpieces. Inknown bending methods, such as for example according to EP 0 341 211 A2,metal sheets are bent by means of a bending device, which comprises apunch as well as a forming die with an adjustable base. The punchpenetrates according to the desired bending angle up to a previouslydefined penetration depth while simultaneously pushing the die basealong the bending line.

It has been established in practice that variations often occur in thebending angle. This has been traced back to the fact that the sheetmetal sides spring back by an undefined amount as soon as the punch hasbeen released.

The undesirable springing back resulted in the development of anadaptive bending method according to DE 694 01 594, in which acorrection value is calculated using a bending formula, in which thebending angle is established during the bending process by means oftracer pins, which are arranged laterally during the bending process onthe workpiece, and by applying the bending formula a correction value isdetermined for the end position of the punch.

The aforementioned methods are complicated in practice and do not allowuniversal applications on any shapes of workpieces and bending angles.Therefore, a simple method has been sought in which the bending anglecan be adjusted precisely and which with high precision makes itpossible to obtain reproducible values for the geometry of the bentworkpiece. In addition, the bending angle should be variable locally inrelation to the width of the metal sheet and thus also the springingback behaviour should be adjustable locally. Furthermore, a bendingdevice suitable for the new method should be structured to be simplerand should be able to be used more flexibly.

The solution to these problems consists according to the inventionessentially in that the stresses created by the bending process in theshaping zone are superimposed by a compressive stress in locally andsequentially consecutive steps, i.e. incrementally. In this case it isassumed that maximum tensile stress is reached in a shaping zonedisplaced into the bend sides, so that here too the superimposition ofstress has to be performed with the applied compressive stress in thisarea of the shaping zone. By controlling this process a localised changeof the bending angle and springing back behaviour should be madepossible in the direction of the sheet metal parts.

In practice, the device according to the invention therefore provides apressure roller, in particular a steel roller with a preferably V-shapedgroove, annular groove or with an elastomer coating underneath thepunch. By means of the pressure rollers compressive stresses in thepulling area of the shaping zone are transmitted along the bending line,which on the one hand do not exceed the value of the flow beginning(R_(P 0,2)) and on the other hand change the stressed state of thebending part locally and in time, so that afterwards the springing backcan be compensated for by more than 50%. For this if necessary a bendingsensor is driven along in front of the pressure roller according to thepath of the bending line on the workpiece and afterwards the advance ofthe nip roller is determined by means of a pressure control.

In principle, it is known from other shaping methods to reduce thespringing back of the sheet metal by means of compressive stresssuperimposition. For example DE 196 08 985 A1 shows a pulling methodwith a divided pressure cushion in which the hold-down plate can bedriven in the manner of a counter-pulling process during the shaping.However, specific steps for compensating the springing back during sheetbending cannot be derived therefrom.

In the following, the invention is explained in more detail withreference to several exemplary embodiments. The initial basis of theinvention is the already mentioned free bending, in which according toFIG. 1 a punch is pushed from above onto a metal sheet. The metal sheetlies horizontally on a forming die, which is provided with two paralleldie assemblies for the bending edges. If the punch is pushed with itspunch tip into the forming die the metal sheet is bent downwards intothe forming die, being supported laterally on the radii of the formingdie edges. Hereby on the inner side of the metal sheet in the region ofthe shaping zone compressive stresses are created, whilst on the outsideof the metal sheet in the region of the shaping zone there are tensilestresses. The metal sheet now has in the loaded state a bending angleα₁. It consists of the lateral bending sides, which run towards thelower punch bending edge.

It is in fact known that the maximum level of tensile stresses is alongthe bending line, however according to the inventor's findings it isimportant to provide compensation by superimposing the tensile stresswith compressive stress not only in this area.

With a nip roller with a V-shaped groove or annular groove thecompressive stress is created by the groove surface which presseslaterally from the outside against the radius area. By means of thismeasure in the shaping zone the compressive stress is compensated for,which is sufficient to be able to remove the bent workpiece afterrelease by the punch as a ready-to-use part.

In principle, the nip roller can also have different forms, such as forexample that of a combination of an annular and V-groove or can have aspherical surface such as for example with a barrel. It is also possibleto provide the nip roller with an elastomer coating.

It is advantageous, if at a distance in front of the nip roller abending angle sensor is inserted between the bending edges. Then theadvance of the nip roller can be pressure controlled depending on themeasurement of the sensor signal but also on the compressive force to beset. However, it is also possible to move the sensor from the outside upto the bending edges, if this is expedient in consideration of the localconditions.

EXAMPLE 1

By means of the advance speed the superimposition of stresses isinfluenced in terms of time. Investigations have shown that good resultscan be obtained with a constant and relatively slow advance speed.

EXAMPLE 2

The geometry of the V-groove is such that the shaping zone is reached inthe bending sides. In this case it is important that there is contactbetween the surface in the region of the groove and the bending parteven outside the radius area.

EXAMPLE 3

The groove width should preferably be greater than 2× the bending radiusor curve shape of the punch, so that with a V-shape the contact surfaceis as large as possible with the pressure roller along the bending line.

EXAMPLE 4

To improve the existing contact in the shaping zone at a constantworking width, the method according to the invention can be performedwith a variable application of pressure. This is a great advantagemainly in the case of metal sheets with different material properties orthicknesses.

1. Method for bending flat metal workpieces, in particular steel sheets,by bending freely over edges along a bending line with straight lineartool movement, wherein the bending edges are arranged parallel to thelongitudinal direction of the sheet and the tool moves along the sheetwidth, characterised by the following steps a) inserting a punch betweenthe bending edges of a forming die, b) holding the punch at the lowestpoint of the bend, c) applying compressive stress to compensate for thestresses in the shaping zone produced by bending d) incrementalsuperimposition of the tensile stress by compressive stress, which isperformed along the bending line in the direction of the sheet width ofthe bent workpiece, whereby the tool generating the compressive stressis moved along the bending line in the direction of the sheet width. 2.Method according to claim 1, characterised in that the superimpositionof the tensile stress in the shaping zone is performed by anincrementally generated compressive stress between the bending edge andbending line, and the bending angle is influenced (adjusted) variablylocally over the width of the metal sheet.
 3. Method according to one ofthe preceding claims, characterised in that the incrementalsuperimposition of the tensile stress with compressive stress isperformed by surface pressure, which is in the elastic range i.e. belowthe yield stress limit R_(P 0,2) of the respective material.
 4. Methodaccording to one of the preceding claims, characterised in that thecompressive stress is generated incrementally with an increasing anddecreasing stress peak in the region of the maximum tensile stressbetween the bending edge and bending line.
 5. Bending device forperforming the method according to claim 1, consisting of a forming diewith two parallel die assemblies and a punch arranged above the dieassemblies, which punch can be inserted into the intermediate spacebetween the bending edges, characterised in that between the dieassemblies underneath the punch a carriage with a nip roller effectivein the punch axis is arranged which can be moved in the direction of thebending axis (i.e. in the sheet width), which nip roller comprises agroove in the section plane with the punch axis.
 6. Bending deviceaccording to claim 5, characterised in that a bending angle sensor canbe inserted at a distance in front of the nip roller between the bendingedges and the nip roller is advanced according to the sensor signalmeasurement.
 7. Bending device for performing the method according toclaim 1, characterised in that the groove has a V-shape, a radius. 8.Bending device according to one of the preceding claims, characterisedin that the forming die comprises interchangeable and rotatable dieassemblies and in this way different die widths can be adjusted. 9.Bending device according to one of the preceding claims, characterisedin that the application of compressive stresses is performedincrementally by means of the nip roller along the bending line. 10.Bending device according to one of the preceding claims, characterisedin that the advancing speed for the movement of the nip roller along thebending line of the bent workpiece is controlled.
 11. Bending deviceaccording to one of the preceding claims, characterised in that thedepth of the V-groove in the nip roller is measured such that thecontact area of the selected groove (V-groove, annular groove) extendswith the external wall of the metal sheet into the pulling area of theshaping zone of the metal sheet.
 12. Bending device according to one ofthe preceding claims, characterised in that the groove width of theV-groove is adjusted to the bending radius of the metal sheet, so that alarge contact area is created with the nip roller over the entire widthof the shaping zone.